Control valves are commonly used in process control systems such as, for example, for controlling product flow in a food processing plant, for maintaining fluid levels in large tank farms, etc. Control valves are used to manage the product flow or to maintain the fluid levels by functioning like a variable passage. The amount of fluid flowing through a valve body of the control valve can be accurately controlled by precise movement of a valve plug (e.g., a valve member) disposed within the valve body. In some known automated control values, such as sliding stem valves (e.g., a gate valve, a globe valve, a diaphragm valve, a pinch valve, etc.), a valve stem operatively couples the valve plug to an actuator. The actuator moves the valve plug within the valve body relative to a seat ring between an open position and a closed position to allow or restrict the flow of fluid between an inlet and an outlet of the valve. The valve stem is connected to the valve plug at an opening of the valve plug and is oriented perpendicular to the valve plug. The alignment between the valve stem and the valve plug is an important factor in ensuring proper sealing interaction between the valve plug and seat ring and achieving tight shut-off performance.
In some known control valves, the valve stem is connected to the valve plug via threads at a threaded opening of the valve plug. However, the tolerances between the threads on the valve stem and threaded opening often result in misalignment. Additionally, the valve stem is typically non-rotatably fixed to the valve plug by using a drive pin, which can cause misalignment between the valve stem and the valve plug and affect the integrity of the valve plug.
Other known control valves use a valve stem having a neck or sharp bevel to assist in aligning the valve stem with the opening in the valve plug. However, manufacturing such a profile requires removing material from a stem stock and, thus, creates weaker sections in the valve stem. Additionally, the neck creates an area where forces tend to concentrate and affect the strength of the valve stem. Further, when known valves are exposed to high temperatures and/or high vibrations, the valve stem can become worn and loosen within the opening of the valve plug. As a result, valve stem and plug connections become misaligned and, thus, require maintenance or replacement.